SLUS872E January   2009  – February 2019 UC1842A-SP , UC1844A-SP

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
    1.     Pin Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Electrical Characteristics (Radiation-Improved Devices)
    7. 7.7 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 UVLO
      2. 8.3.2 Reference
      3. 8.3.3 Totem-Pole Output
    4. 8.4 Device Functional Modes
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Oscillator
        2. 9.2.2.2 Current Sensing and Limiting
        3. 9.2.2.3 Error Amplifier
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Feedback Traces
      2. 11.1.2 Input/Output Capacitors
      3. 11.1.3 Compensation Components
      4. 11.1.4 Traces and Ground Planes
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Related Links
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

9.2.2.1 Oscillator

The UC184xA-SP oscillator is programmed as shown in Figure 15. Timing capacitor CT is charged from VREF (5 V) through the timing resistor RT, and discharged by an internal current source. The first step in selecting the oscillator components is to determine the required circuit dead time. Once obtained, Figure 16 is used to pinpoint the nearest standard value of CT for a given dead time. Next, the appropriate RT value is interpolated using the parameters for CT and oscillator frequency. Figure 17 shows the RT/CT combinations versus oscillator frequency. The timing resistor can be calculated from the following formula.

Equation 1. UC1842A-SP UC1844A-SP eqn1_slusci6.gif
UC1842A-SP UC1844A-SP sch_EA_comp_LUSC14.gifFigure 14. E/A Compensation Circuit for Continuous Boost and Flyback Topologies

The UC184xA-SP has an internal divide-by-two flip-flop driven by the oscillator for a 50% maximum duty cycle. Therefore, their oscillators must be set to run at twice the desired power supply switching frequency.

UC1842A-SP UC1844A-SP oscillator_LUSC14.gifFigure 15. Oscillator Programming
UC1842A-SP UC1844A-SP gr_dt_vs_Ct_lusc14.gifFigure 16. Dead Time vs CT (RT > 5 kΩ)
UC1842A-SP UC1844A-SP gr_RT_freq_lusc14.gifFigure 17. Timing Resistance vs Frequency